Mode-locking of semiconductor laser diodes is of interest as such sources have applications in telecommunications and in measurement systems.
Mode-locking of laser signals is used to produce trains of short pulses. Mode-locking is achieved when all modes of oscillation in a laser cavity have the same phase and when the frequency difference between neighboring oscillation modes is a constant throughout the entire emission spectrum. Under these conditions the laser signal in the time domain is defined by the coherent superposition of all oscillating, phase-locked lasing modes in the laser cavity. The main characteristics of the laser signal when mode-locking is achieved are:    i. Constructive interference of all oscillating lasing modes takes place over a short period of time, giving rise to a sharp emission spike (or a pulse). The duration of the pulse has an inverse proportional relationship with the number of oscillating lasing modes.    ii. Destructive interference between all oscillating lasing modes takes place at all other instants, producing a weak or a vanishingly small output signal. A residual level of emission in the output signal is caused by an intrinsic noise due to spontaneous emissions in the laser medium; hence the residual signal is composed of only a few photons.    iii. The laser signal is periodic, with a period defined by a duration of a round-trip wave propagation in the laser cavity.
To operate a semiconductor laser diode in the mode-locked regime, one needs to implement a modulation mechanism where the losses in the lasing medium are modulated periodically, with a period equal to the duration of a round-trip wave propagation in the laser cavity. There are general methods which are used to produce such a modulation of which: active modulation and passive modulation are but two examples. A combination these two methods has been used; under such circumstances the regime of operation is called hybrid mode-locking.
Mode-Locked Semiconductor Laser Diodes with Active Modulation
Direct modulation of the laser gain is widely used.
One method of actively modulating a semiconductor laser diode is achieved by inserting a modulator in the laser cavity associated with the semiconductor laser diode. A modulation period of the modulator is adjusted to be substantially equal to a duration of a round-trip wave propagation in the cavity or, sometimes, to a submultiple of the round-trip duration. The modulator has a 100% transmission only over a short period of time, and quenches laser signal travelling therethrough at all other instants.
In general the semiconductor laser diode is operated with an extended cavity, in order to physically accomplish the insertion of the modulator and to match the round-trip wave propagation time of the laser cavity to the modulation period of available modulators. An extended cavity is also used, in order to match the round-trip wave propagation time to the period of modulating sources available to generate the modulating electrical signals.
Another method of actively modulating a semiconductor laser diode is achieved by directly modulating the gain of the semiconductor laser diode. A coupling of a periodic electrical signal to the input of the semiconductor laser diode provides the gain necessary for laser oscillation. A period of the electrical signal applied to the laser diode is matched to the round-trip wave propagation time period in a laser cavity associated with the laser diode. As a result the gain is pulsed at a modulation period corresponding to the duration of the round-trip wave propagation in the laser cavity. Gain dynamics lead to the development of short pulses, which repeat at the modulation period.
References: S. W. Corzine, J. E. Bowers, G. Przybylek, U. Koren, B. I. Miller, and S. E. Socoolich, “Actively Mode-locked GaInAsP Laser with Subpicosecond Output”, Appl. Phys. Lett. 52, 348 (1988); A. Mar, D. Derickson, R. Helkey, J. Bowers, R.-T. Huang, and D. Wolf, “Actively Mode-locked External-cavity Semiconductor Lasers with Transform-limited Single-pulse Output”, Opt. Lett. 17, 868 (1992); A. Azouz, N. Stelmakh, P. Langlois, J.-M. Lourtioz, and P. Gavrilovic, “Nonlinear Chirp Compensation in High-power Broad-spectrum Pulses from Single-stripe Mode-locked Laser Diodes”, IEEE J. Selected Topics in Quantum Electron. 1, 577 (1995); and R. S. Tucker, U. Koren, G. Raybon, C. A. Burrus, B. I. Miller, T. L. Koch, and G. Eisenstein, “40 GHz Active Mode Locking in a 1.5 μm Monolithic Extended-cavity Laser”, Electron. Lett. 25, 621 (1989) represent developments in active modulation mode-locking techniques. A key feature of actively mode-locked operation is the possibility of synchronizing the pulses with respect to a reference electrical signal derived from the modulation signal.
Mode-Locked Semiconductor Laser Diodes with Passive Modulation
Passive modulation has been generally achieved to date using saturable absorbers. A saturable absorber has a property that its transmission is controlled by an incident laser beam. Due to the dynamics of the energy levels involved in the absorption process, transmission increases as the laser signal intensity increases. As a result, the peak of a pulse propagating therethrough undergoes a smaller absorption than the wings. The net effect is that the transmitted pulse is shortened with respect to the incident pulse. Furthermore saturable absorbers tend to quench weak signals.
Hence saturable absorbers provide a mechanism of self-modulation of the laser signal. When inserted in a laser cavity, saturable absorbers tend to restrict laser emission to short pulses. The technique is said to be passive since no external electrical or optical signal is necessary to produce the modulation.
In some cases such as presented in E. P. Ippen, D. J. Eilenberg, and R. W. Dixon, “Picosecond Pulse Generation by Passive Mode Locking of Laser Diodes”, Appl. Phys. Lett. 37, 267 (1980), saturable absorption results from a progressive degradation of the lasing medium or from an optical damage (e.g. a damage induced by the circulating laser beam). Fulfilling such conditions generally leads to an irreversible damage imparted to and eventual inoperability of the laser diode.
A saturable absorber can be created by ion implantation at one facet of a laser diode, as described in J. P. Van der Ziel, W. T. Tsang, R. A Logan, R. M. Mykuliak, and W. M. Augustyniak, “Subpicosecond Pulses from Passively Mode-locked GaAs Buried Optical Guide Semiconductor Lasers”, Appl. Phys. Lett. 39, 525 (1981), and N. Stelmakh and J.-M. Lourtioz, “230 fs, 25 W Pulses from Conventional Mode-locked Laser Diodes with Saturable Absorber Created by Ion Implantation”, Electron. Lett. 29, 160 (1993).
Multiple quantum well structures can also be used as saturable absorbers in extended cavities, as described in Y. Silberberg, P. W. Smith, D. J. Eilenberger, D. A. B. Miller, A. C. Gossard, and W. Wiegmann, “Passive Mode Locking of a Semiconductor Laser Diode”, Opt. Lett 9, 507 (1984).
Non-uniform current injection along the laser diode can also be used for passive mode-locking, as reported in C. Harder, J. S. Smith, K. Y. Lau, and A. Yariv, “Passive Mode Locking of Buried Heterostructure Lasers with Non-uniform Current Injection”, Appl. Phys. Lett. 42, 772 (1983), and Y. K Chen, M. C. Wu, T. Tanbun-Ek, R. A. Logan, and M. A. Chin, “Subpicosecond Monolithic Colliding Pulse Mode-locked Multiple Quantum Well Lasers”, Appl. Phys. Lett. 58, 1253 (1991).
When the latter procedure is used, the gain medium is divided in many sections, some of which are not polarized or are set in reverse biased conditions; saturable absorption takes place in sections that are not polarized or that are operated with a reverse bias.
Passive mode-locking has enabled the generation of subpicosecond pulses, at typical average powers of around 1 mW. When a saturable absorber is placed at center of a monolithic structure as described by Y. K Chen et. al. in the publication mentioned above, or at the center of a laser diode placed in an external ring cavity as described in C. F. Un and C. L. Tang, “Colliding Pulse Mode Locking of a Semiconductor Laser in an External Ring Cavity”, Appl. Phys. Lett 62, 1053 (1993), the laser diode tends to emit a pair of counterpropagating pulses which collide in the saturable absorber. Such a regime of emission is called “colliding pulse mode-locking” (CPM). CPM is generally considered to be more stable since the saturable absorption effect is enhanced by the superposition of the counterpropagating pulses.
Passive modulation has the advantage of simplicity when compared to active modulation, as the matching of the round-trip wave propagation period to an external modulation signal is not required. In general, passive mode-locking leads to shorter pulses than active mode-locking. However the pulses produced through passive mode-locking cannot be synchronized with respect to any signal.
Hybrid Mode-Locking of Semiconductor Laser Diodes
Hybrid mode-locking involves the combination of active and passive modulation. Hybrid mode-locking has been adapted to monolithic structures, such as described in M. C. Wu, Y. K. Chen, T. Tanbun-Ek, R. A. Logan, and M. A. Chin, “Transform-limited 1.4 ps Optical Pulses from a Monolithic Colliding-pulse Mode-locked Quantum Well Laser”, Appl. Phys. Lett. 57, 759 (1990), or in extended cavities, such as described in P. J. Delfyett, L. Florez, N. Stoffel, T. Gmitter, N. Andreadakis, G. Alphonse, and W. Ceislik, “200-fs Optical Pulse Generation and Intracavity Pulse Evolution in a Hybrid Mode-locked Semiconductor Diode-laser/Amplifier System”, Opt. Lett. 17, 670 (1992). One can benefit from the advantages of both modulation techniques, but at the price of an added complexity.
Other Mode-Locking Methods
Solid-state lasers, such as color center and titanium-sapphire lasers, have been mode-locked through a variety of techniques that involve nonlinear phase modulation (self-phase modulation, self-focusing). Early reports on that subject can be found in L. F. Mollenauer and R. H. Stolen, “The Soliton Laser”, Opt Lett. 9, 13 (1984), and D. E. Spence, P. N. Kean, and W. Sibbett, “60-fsec Pulse Generation from a Self-mode-locked Ti:sapphire Laser”, Opt. Lett. 16, 42 (1991).
To date, only one method relying on nonlinear phase modulation has been successfully tested with semiconductor laser diodes: coupled-cavity mode-locking. According to the results presented in E. M. Dianov and O. G. Okhotnikov, “Coupled-cavity Passive Mode-locked Injection Laser”, IEEE Photon. Technol. Lett. 3, 499 (1991), and W. H. Loh and C. F. Lin, “Optical Pulse Generation with a Semiconductor Laser in a Coupled-cavity Configuration”, Pure Appl. Opt. 1, 181 (1992), coupled-cavity mode-locking has produced pulses whose duration was much longer when compared to the best results obtained with either active, passive or hybrid mode-locking.
In solid-state lasers the most efficient method of generating trains of short pulses is Kerr-lens mode-locking as described by D. E. Spence et. al. in the above mentioned publication. This method provides short stable and controllable pulses having a fast repetition rate and high peak power. Kerr-lens mode-locking relies upon self-focusing, e.g. on a lensing effect produced by the laser pulse itself when it propagates through the gain medium. Kerr-lens mode locking takes place only when the cavity geometry is set such that non-linear tensing improves round-trip laser gain or decreases round-trip losses.
In guided-wave structures such as semiconductor laser diodes, self-focusing is not effective as a mode-locking mechanism since it represents only a weak effect in comparison with the guiding effect produced by the guided-wave structure itself. This is probably why passive self-modulation mode-locking, based upon self-focusing is not operational in semiconductor laser diodes.
Therefore there is a need to develop novel methods of providing sustained passive self-modulated mode-locked operation of a semiconductor laser diode.